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Guest essay by Eric Worrall

Why bother with capturing concentrated CO2 from combustion exhaust, when you can spend “a wartime level of funding” capturing trace concentrations of CO2 directly out of the air?

Engineers have built machines to scrub CO₂ from the air. But will it halt climate change?

January 21, 2021 6.08am AEDT

Deanna D’Alessandro
Professor & ARC Future Fellow, University of Sydney

US research published last week suggested global warming could be slowed with an emergency deployment of a fleet of “CO₂ scrubbers” using DAC technology. However a wartime level of funding from government and business would be needed. So is direct air capture worth the time and money?

What’s DAC all about?

Direct air capture refers to any mechanical system capturing CO₂ from the atmosphere. Plants operating today use a liquid solvent or solid sorbent to separate CO₂ from other gases.

Swiss company Climeworks operates 15 direct air capture machines across Europe, comprising the world’s first commercial DAC system. The operation is powered by renewable geothermal energy or energy produced by burning waste.

Canadian company Carbon Engineering uses giant fans to pull air into a tower-like structure. The air passes over a potassium hydroxide solution which chemically binds to the CO₂ molecules, and removes them from the air. The CO₂ is then concentrated, purified and compressed.

DAC technology is currently expensive, relative to many alternative ways of capturing CO₂, but is expected to become cheaper as the technology scales up. The economic feasibility will be helped by the recent emergence of new carbon markets where negative emissions can be traded.

DAC machines process an enormous volume of air, and as such are very energy-intensive. In fact, research has suggested direct air capture machines could use a quarter of global energy in 2100. However new DAC methods being developed could cut the technology’s energy use.

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Potassium hydroxide, a potent form of hardware store lye, is pretty good at grabbing CO2 out of the air. The difficult part is getting the potassium to let go of the CO2 it just grabbed, and regenerating the lye.

The “new DAC methods” use organic metal compounds, kind of like how your body uses metal organic haemoglobin in your blood to transport oxygen. Incorporating metal into an organic compound in principle lets you engineer the metal’s CO2 grabbing tendencies to better match requirements. But organic metal compounds tend to be fragile, easily damaged by contaminants or regular wear and tear – your body devotes a lot of resources to replacing damaged haemoglobin. So it will be interesting to see how this new organic metal compound process handles real world conditions.

Regardless of the exact process selected I doubt this approach to reducing CO2 will make a significant difference to atmospheric CO2 in the foreseeable future. The idea of grabbing CO2 directly from the air just seems absurd when there are far more efficient alternatives available. If you want to spend taxpayer’s money setting up a CO2 mine, surely it would make more sense to grab your CO2 from a concentrated source like a big industrial smokestack, or grow a few more trees, rather than setting up an industrial process to try to filter trace amounts of CO2 from the atmosphere.

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